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International Conference on Ultrafast Phenomena

Wave packet interference in the free induction decay from molecules

John N. Sweetser, Thomas J. Dunn, Ian A. Walmsley

Published Jan 1, 1994• 0 citations
Publisher

Abstract

It is well known that the application of a pulse to a molecular electronic transition leaves the molecules vibrational modes in different electronic states in well-localized states, or wave packets[1]. One method of detecting the wave packets is via the time-dependent spectra of spontaneous emission from the excited molecule[2]. The emission consists in general of two parts, an incoherent part that depends only on the wave packet in the excited state, and a coherent part, that depends on the wave packets in both excited and ground electronic states (of a two-state molecule). This can be seen from the following simple calculation.

DOI
10.1364/up.1994.wc.15

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It is well known that the application of a pulse to a molecular electronic transition leaves the molecules vibrational modes in different electronic states in well-localized states, or wave packets[1]. One method of detecting the wave packets is via the time-dependent spectra of spontaneous emission from the excited molecule[2]. The emission consists in general of two parts, an incoherent part that depends only on the wave packet in the excited state, and a coherent part, that depends on the wave packets in both excited and ground electronic states (of a two-state molecule). This can be seen from the following simple calculation.
The present thesis is devoted to theoretical studies of pulse propagation of light through linear and nonlinear media, and of light-induced nuclear dynamics. The first part of the thesis addresses propagation of light pulses in linear periodical media - photonic crystals. The main accent was put on studies of the angular properties of two qualitatively different types of photonic crystals: holographic photonic crystals, and impurity band based photonic crystals. The anisotropy of band structure, group velocity and pulse delay with respect to the light polarization are analyzed. In the second part of the thesis a strict theory of nonlinear propagation of a few strong interacting light beams is presented. The key idea of this approach is a self-consistent solution of the nonlinear wave equation and the density matrix equations of the material. This technique is applied to studies of dynamics of cavityless lasing generated by ultra-fast multi-photon excitation. It is shown that interaction of co- and counter-propagating pulses of amplified spontaneous emission (ASE) affects the dynamics and efficiency of nonlinear conversion. Our dynamical theory allows to explain the asymmetric spectral properties of the forward and backward ASE pulses, which were observed in recent experiment with different dye molecules. It is shown that the ASE spectral profile changes drastically when the pump intensity approaches the threshold level. The effect of the temporal self-pulsation of ASE is studied in detail. The third part of the thesis is devoted to light-induced nuclear dynamics. Time- and frequency-resolved X-ray spectroscopy of molecules driven by strong and coherent infrared (IR) pulses shows that the phase of the IR field strongly influences the trajectory of the nuclear wave packet, and hence, the X-ray spectrum. Such a dependence arises due to the interference of one (X-ray) and two-photon (X-ray + IR) excitation channels. The phase of the light influences the dynamics also when the Rabi frequency approaches the vibrational frequency, breaking down the rotating-wave approximation. The probe X-ray spectra are also sensitive to the delay time, the duration, and the shape of the pulses. The evolution of the nuclear wave packets in the dissociative core-excited state affects the dynamics of resonant Auger scattering from fixed-in-space molecules. One of the important dynamical effects is the atomic-like resonance which experiences electronic Doppler shift. We predict that the scattering of the Auger electrons by nearby atoms leads to new Doppler shifted resonances. These extra resonances show sharp maxima in the bond directions, which makes them very promising as probes for local molecular structure using energy and angular resolved electron-ion coincidence techniques. Our theory provides prediction of several new effects, but also results that are in good agreement with the available experimental data.
We present a time- and phase-resolved, background-free scheme to study the extreme ultraviolet dipole emission of a bound electronic wave packet, without the use of any extreme ultraviolet exciting pulse. Using multiphoton transitions, we populate a superposition of quantum states which coherently emit extreme ultraviolet radiation through free induction decay. This emission is probed and controlled, both in amplitude and phase, by a time-delayed infrared femtosecond pulse. We directly measure the laser-induced dephasing of the emission by using a simple heterodyne detection scheme based on two-source interferometry. This technique provides rich information about the interplay between the laser field and the Coulombic potential on the excited electron dynamics. Its background-free nature enables us to use a large range of gas pressures and to reveal the influence of collisions in the relaxation process.